The present invention relates to a photographic lens device of the type in which a step motor is employed to drive an aperture-stop setting means or its blades.
In the photographic lens devices of the type described, in response to an electrical control signal from a camera body, the step motor is driven to drive a plurality of iris type blades to set a predetermined aperture stop. A variety of such photographic lens devices have been so far devised and demonstrated, but the motor used is of the so-called power motor; that is, a motor in which the rotor substantially occupies the space defined by the stator and the power is transmitted through the shaft of the rotor. As a result, the power motor must be disposed at a suitable space outside of the lens barrel; that is, the light path through the lens system, so that the photographic lens device becomes enlarged in size especially in the radial direction. Therefore, some photographic lens barrels are not cylindrical in shape.
The same inventors proposed an aperture stop control system in which a step motor drives a plurality of blades as is disclosed in U.S. Pat. No. 4,005,448, granted to the same applicant even though this system was not shown as being applied to the photographic lens device. This system is also capable of controlling a shutter speed. The step motor employed is featured in that its rotor is in the form of a hollow cylinder and is operatively coupled to a plurality of blades which are disposed within the rotor. Therefore, the light rays can pass through the ring-shaped rotor and an aperture stop defined by the blades. The system can be used as an exposure control system.
The step motor of the type described can eliminate an intermediate transmission means which is extended along the axis of rotation of the rotor so as to connect it to the blades. To put it another way, the hollow cylindrical rotor can be directly connected to the blades. In addition, the light rays are permitted to pass through the step motor. Therefore, it becomes possible to provide an exposure control system which is cylindrical in shape.
Photographic lens devices are in general extremely high precision products including a mechanism for converting the rotation of a focusing ring into the straight translation of a lens barrel or a photographic lens system to be referred to as "a helicoidal mechanism" in this specification. It follows, therefore, that it is almost impossible to freely change their designs, and consequently; the position of the exposure control system of the type described in the photographic lens device is limited. In general, a set of aperture-stop control blades is interposed between lens elements and it is, of course, most preferable to not change its position even when a step motor or the like for driving the blades is disposed within the photographic lens device so that photographic lens devices with a built-in, motor-driven aperture-stop control system may be substantially similar in shape to the conventional photographic lens devices.
In the previously proposed aperture-stop control system of the type described, a set of blades is disposed very closely to the rotor of a step motor through a coupling or driving means and radially inwardly of the rotor. Accordingly, in order to build the conventional aperture-stop control system into a space for containing aperture blades between two lens elements in the lens barrel, the space has to be expanded in the axial direction because there is no space to contain the rotor of the step motor in the conventional lens barrel. As a result, the lens barrel has to be extended in the axial direction, which is undesirable for cameras.